1. Field of the Invention
The present invention relates to a heat-conductive aluminum nitride sintered body and a method of manufacturing the same. More particularly, the invention relates to a dense heat-conductive aluminum nitride sintered body having desirable characteristics such as an excellent heat conductivity, a high insulation resistance and permittivity or dielectric constant. A method of manufacturing the present body is also disclosed.
2. Description of the Background Art
Large scale integrated circuit (LSI) techniques have been remarkably advanced in recent years, particularly in improving the circuit density. An increase in chip size of a semiconductor integrated circuit (IC) also contributes to such improvement in the circuit density. The calorific power of a package for carrying an IC chip is increased with the increase in the IC chip size. Thus, importance has been attached to the heat radiation or dissipation of a material for an insulator substrate, which is applied in a package for a semiconductor device or the like. Such an insulator substrate is generally made of alumina (Al.sub.2 O.sub.3). However, alumina has a small thermal conductivity of 30 W/mK, whereby alumina is inferior in its heat dissipation ability compared to its excellence in electrical insulation resistance and mechanical strength. Hence, it is inadequate to mount a field-effect transistor (FET) of high calorific power or the like, on an alumina substrate. Although there is an insulator substrate which is made of beryllia (BeO) having a high thermal conductivity for supporting a semiconductor element of high calorific power, safety measures for using such a substrate are complicated since beryllia is toxic.
With respect to an insulator substrate for supporting a semiconductor element of high calorific power, a heat-conductive aluminum nitride (AlN), which is non-toxic and equivalent to alumina in its electrical insulation resistance and mechanical strength, has been recently seen to be an effective insulating material for a semiconductor device or a package for such a device.
As hereinabove described, aluminum nitride theoretically has a high thermal conductivity and insulation resistance as a monocrystal material. However, since powder of aluminum nitride itself is inferior in its ability to sinter, the relative density of a sintered body prepared by forming such aluminum nitride powder and sintering the powder is about 70 to 80% at the most, of the theoretical density of aluminum nitride of 3.26 g/cm.sup.3, depending on sintering conditions, since the sintered body contains a large amount of pores. Therefore, it is difficult to densify an aluminum nitride sintered body by independently employing aluminum nitride powder.
In an insulating ceramics member such as an aluminum nitride sintered body, thermal conduction takes place mainly through phonon conduction. Thus, phonon scattering is caused by defects such as pores, impurities, etc. contained in the sintered body, to reduce the level of its thermal conductivity. In order to obtain an aluminum nitride sintered body having high thermal conductivity, various proposals have been made as follows:
(1) Japanese Patent Laying-Open Gazette No. 96578/1985, for example, discloses a method of adding Y.sub.2 O.sub.3, serving as a sintering assistant and a deoxidizer, to aluminum nitride powder and sintering the same.
(2) Each of Japanese Patent Laying-Open Gazettes Nos. 71576/1985 and 155263/1986, for example, discloses a method of adding carbon to aluminum nitride powder and sintering the same for deoxidation, thereby to obtain a sintered body having a small content of oxygen.
(3) Each of Japanese Patent Laying-Open Gazettes Nos. 71575/1985 and 127267/1985, for example, discloses a method of employing high purity aluminum nitride powder having a small content of oxygen.
(4) Japanese Patent Laying-Open Gazette No. 41766/1987, for example, discloses a method of decomposing/evaporating a sintering assistant remaining in a sintered body, to obtain an aluminum nitride sintered body of high purity.
(5) Proceedings of the 1987 Yogyo Kyokai's Annual Meeting p. 969, for example, discloses a method of removing a sintering assistant remaining in a sintered body by exposing the same to a reducing atmosphere for a long time.
In the above method (1) of adding Y.sub.2 O.sub.3 and performing ball-mill mixing, it is necessary to add at least 1 percent by weight of Y.sub.2 O.sub.3 in order to obtain a dense heat-conductive sintered body. However, the thermal conductivity of a sintered body thus obtained is about 200 W/mK at the most, and dispersion thereof is significant. If a large amount of Y.sub.2 O.sub.3 is added, thick intergranular phases are formed around aluminum nitride particles, to reduce oxidation resistance of the sintered body while increasing the permittivity.
The above method (2) reduces the amount of oxygen contained in aluminum nitride through a deoxidizing action by the carbon. However, when no sintering assistant is added, the sintered body has a low thermal conductivity of about 80 W/mK since it is difficult to densify the low density of the sintered body as disclosed in Japanese Patent Laying-Open Gazette No. 71576/1985. If Y.sub.2 O.sub.3 is to be added as a sintering assistant as disclosed in Japanese Patent Laying-Open Gazette No. 155263/1986, it is necessary to add at least 1 percent by weight of Y.sub.2 O.sub.3 in order to density the sintered body.
According to Japanese Patent Laying-Open Gazette No. 71575/1985 listed in the above item (3), the amount of oxygen contained in the sintered body is at least 0.5 percent by weight while the sintered body has a low thermal conductivity of about 40 W/mK, even if aluminum nitride powder of high purity is employed as a raw material. As shown in Japanese Patent Laying-Open Gazette No. 127267/1985, the thermal conductivity of a sintered body obtained by adding at least 3 percent by weight of Y.sub.2 O.sub.3, is about 70 W/mK.
As hereinabove described, an aluminum nitride sintered body obtained by adding a small amount of sintering assistant has a low thermal conductivity under the above described circumstances.
According to the above method (4) or (5) of adding a large amount of sintering assistant, sintering the material and thereafter removing the sintering assistant, it is assumed that an aluminum nitride sintered body thus obtained has high thermal conductivity exceeding 200 W/mK including an improvement in permittivity etc. However, according to a method of removing the sintering assistant by adding a fluoride, the sintering furnace is inevitably contaminated and differences in the characteristics of sintered bodies thus obtained, are increased not only in a lot of sintered bodies but even in the same sintered body, as shown in Japanese Patent Laying-Open Gazette No. 41766/1987. Further, the surface of the sintered body is so roughened that an after-treatment must inevitably be used, because of surface irregularities caused by evaporation and volatilization of the sintering assistant. According to the method of sintering the material in a reducing atmosphere for a long time as disclosed in "Proceedings of the 1987 Yogyo Kyokai's Annual Meeting", the costs of such sintering are much increased while the sintering assistant segregates in the sintered surface of the sintered body. Thus, the sintered body cannot be directly employed with such a rough sintered surface, while its characteristics vary within a large range.